1. Field of the Invention
This invention relates to a rake combiner, in particular a rake combiner for a CDMA rake receiver.
2. Description of the Prior Art
Multiple access communication systems allow a large number of users to establish wireless communication channels over a relatively limited frequency spectrum. One multiple access communication system that has become increasingly prominent is the spread spectrum multiple access telecommunication system, otherwise known as code division multiple access (CDMA).
Multiple access in a CDMA system is achieved by assigning each user in the system a pseudo-random code, where the assigned pseudo random codes have good auto and cross-correlation properties. In use the assigned pseudo-random code is modulated with a user's hit rate signal. The bandwidth of the modulated signal incorporating the pseudo-random code is much larger than the bandwidth of the user's bit rate signal, thereby spreading the user's relatively narrowband signal into a wide-band spread spectrum signal.
On receipt of the signal by a designated receiver the wideband signal is converted back into a narrow band signal using the original pseudo-random code to ‘de-spread’ the signal.
By ensuring the pseudo-random codes have good auto and cross correlation properties ‘spread’ signals from other users remain as wideband signals, thereby minimizing interference with the required ‘narrow’ band signal.
An advantage of a CDMA system is its ability to allow separate multipath signals to be combined using a rake receiver. Multipath signals arise from reflections of a signal from obstacles in the environment. The multipath signals are copies of the same transmitted signal but typically have different amplitudes, phases and delays. The time delay difference between the first and the last received multipath component is typically known as the maximum delay spread of the multipath signals.
A rake receiver has a plurality of rake fingers where each finger is allocated to receive a designated multipath component. To equalize the delays between the different multipath components etch finger has a delay equalization memory. The memory in each finger is used to store respective multipath components (i.e. the first multipath component received by a rake finger is stored in the received rake finger's memory until the last multipath component is received by a different rake finger). The delayed equalized multipath components are then combined via a rake combiner and written to memory before being decoded.
However, the use of memory in each rake finger to perform delay equalization can result in an increase in cost and complexity of the rake receiver. Further, the use of memory in each rake ringer to perform delay equalization limits the combination of multipaths to those that are shorter than the maximum delay of the rake finger memory.
It is desirable to improve this situation.